1. Field of the Invention
The present invention relates generally to semiconductor manufacturing and more specifically to methods for detecting transitions of wafer surface properties in chemical mechanical polishing for process status and control.
2. Description of the Related Art
During semiconductor manufacturing, integrated circuits are defined on semiconductor wafers by forming various patterned layers over one another. These patterned layers disposed one over the other define a topography of a surface of the wafer. The topography becomes irregular, i.e., non-uniform (or inhomogeneous), during manufacture. These irregularities present problems during subsequent processing operations, especially in operations for printing a photolithographic pattern having small geometries, for example. The cumulative effects of the irregularities of the topography can lead to device failure and poor yields if the surface topography is not smoothed.
Planarization is used for smoothing the irregularities. One type of planarization is known as chemical mechanical polishing (CMP). In general, CMP processes involve holding and rotating the wafer, and urging the rotating wafer against a polishing pad. An abrasive liquid media (slurry) is applied to the pad to assist in the polishing. A problem encountered during CMP operations is the determination of a “status” during the CMP process. The status may be that a desired flatness of the topography has been achieved, or that there is a desired thickness of material remaining on the surface of the wafer. Other examples of such status relate to the composition of the processed material, e.g., that certain materials have been removed from the wafer so that, for example, certain material in a desired pattern remains as part of an exposed surface of the wafer. Additionally, the status may be that another point of processing has been attained, for example, clearance of overburden material. Also, such status may be that there is a change in the resistance of the processed material.
Each such status relates to a property of the semiconductor wafer and the films on the wafer. The properties may include, for example, topographical, thickness, composition of materials, reflectivity, resistivity, and film quality.
Prior methods of making such status determinations include removing the semiconductor wafer from processing equipment to facilitate stand-alone inspection metrology. Also, as described below, in-situ methods have been provided, and use laser interferometry or broad band spectroreflectometry to monitor the properties of the wafer surface without removing the wafer from the equipment. Also as described below, vibration sensors have been mounted on a head that carries a wafer carrier plate, such that the sensor on the head is located remotely from the wafer.
In-situ methods, such as laser interferometry or spectroreflectometry, typically require an ability to observe the wafer surface through the polishing pad, normally through a specially inserted window. FIG. 1A schematically illustrates a prior in-situ apparatus for measuring a thickness property of a layer of a wafer 102. The wafer 102 is supported on a carrier 104 that is rotated. During CMP operations the wafer 102 is pressed against a pad 106 in the presence of a slurry to planarize a surface 107 of the wafer 102. The pad 106 is supported by a platen 108. A window 110 in the platen 108 and the pad 106 allow a beam from a laser 112 to view the surface 107 of the wafer 102. The pad 106 and the platen 108 may rotate around an axis as illustrated by arrow 114, and the carrier 104 rotates the wafer 102 around an axis as illustrated by arrow 116 as the pad 106 and the platen 108 rotate. European Patent Nos. EP 0,738,561 A1 and EP 0,824,995 A1 discuss in detail a laser interferometer and are hereby incorporated by reference.
A problem encountered with in-situ monitoring of CMP operations is that the environment in a gap 118 between the surface 107 of the wafer 102 and the window 110 contribute to spectral signal variations which typically have changing optical properties due to the dynamic environment and the abrasive nature of the CMP process and due to deposition of process by-products. Slurry and residue from the wafer 102 and the pad 106, as well as air bubbles from turbulence, also contribute to the optical variations caused by the environment of the gap 118. For example, at the initiation of the CMP process the gap 118 is filled with slurry having certain optical characteristics, and calibrations are performed based on such initial optical characteristics. However, as the wafer 102 is planarized the slurry contains increasing percentages of residue from the wafer 102 and the pad 106. Such residue changes the optical characteristics of the slurry in the gap 118, which in turn subjects the measurement of the thickness property to errors. The errors occur when an endpoint detector associated with the laser 112 is calibrated based on those initial optical characteristics of only the slurry or fluid in the gap 118, and when the optical characteristics change for reasons other than the thickness property. While the window 110 may be located at different heights within the pad 106, a gap 118 will always exist so that the window 110 does not come into contact with the wafer 102. U.S. Pat. No. 6,146,242 describes an optical endpoint window disposed under a window in the polishing pad and is hereby incorporated by reference.
Such in situ monitoring is also subject to other limitations. Typically, the location of the window 110 in the platen 108 only periodically overlaps the wafer 102 as the wafer 102 and the platen 108 rotate on the respective axes. As a result, the window 110 in the platen 108 acts as a shutter so that the laser 112 does not constantly illuminate the wafer 102. Also, the shutter action only allows a periodic response by optical devices that receive the laser light reflected from the wafer 102.
In view of these limitations of in-situ monitoring of CMP operations, attempts have been made to sense vibrations during CMP operations. However, referring to FIG. 1B, because typical vibration sensors 130 have been mounted on a head 132 remotely from an interface 134 between a wafer 136 and a pad 138, there is significant mechanical structure between the wafer-pad interface 134 and the sensor 130. Such structure may include a wafer carrier plate 140 and a connector 142 that joins the carrier plate 140 to a rotary drive 144. The wafer carrier plate 140 and the connector 142 interfere with the transmission of vibrations (see arrow 146) from the interface 134. As a result, vibrations (see arrows 148) resulting from the physical characteristics of such structure are more strongly received by the sensor 130, as compared to the vibrations 146 based on the wafer properties at the wafer-pad interface 134 at which the remotely located CMP process takes place. Thus, the process vibrations 146 tend to be dampened as they travel to the remotely located sensor 130. Further, such vibrations 146 are weak in comparison to the vibrations 148 resulting from the physical characteristics of the structure, there tends to be a loss of resolution from the CMP process vibrations 146, and there may be a low signal-to-noise ratio with respect to the process vibrations 146. As a result, the remote sensor 130 tends to output signals that do not accurately indicate the wafer properties at the wafer-pad interface 134, hence the status of the CMP processing may not be accurately indicated. Therefore, control of the CMP process using such inaccurate output signals also tends to be inaccurate.
These limitations of the prior in-situ monitoring, and of the prior vibration sensing, for example, have caused problems in detection of status transitions, or transitions, which are important and characteristic changes in the surface properties of the wafer surface or of the films occurring in a pad/wafer interaction interface and at the wafer surface during CMP processing of the wafer.
What is needed then is a method for detecting the transitions in the wafer and film properties. Such need is to detect such transitions while avoiding the limitations of optical systems that view the wafer through the polishing pad. Therefore, there is a need in such polishing for inspection methods which constantly observe the properties of the polishing surface and/or of a parameter linked to the pad/wafer interface, for detecting any such occurring transitions. Further, there is a need for CMP process status and control methods in which the properties of the wafer surface are sensed at the closest proximity to the wafer, most preferrably within the wafer carrier plate rather than remotely as in the prior remote vibration sensors. A related need is to provide an improved way of sensing parameter variations that reflect the changes in the properties occurring in the wafer/pad interaction interface and/or at the wafer surface. Such improved way should avoid dampening the process-based vibrations before such vibrations are sensed, should result in strong reception of the process vibrations in comparison to vibrations based on the physical characteristics of the structure, should provide a gain in resolution, and should improve the signal-to-noise ratio with respect to the process vibrations. In addition, there is a need for increasing the amount of wafer area that is sensed, so as to sense changes in different properties at different areas of the wafer surface, as compared to the relatively small wafer surface areas sensed by most of conventional in-situ sensors, for example.